Big Bird 88

Need some help with this. I think I have diagnosed an AC heater control valve or Heater control valve actuator issue. Here is what I am seeing.
AC is fully charged and confirmed not leaking.
Compressor is working fine. Blower working fine.

Vehicle running, AC on, Auto / Auto and its blowing nice and cold. Then I hear a faint click, like its coming from behind the AC control head unit or in that vicinity. Now I get HOT air blowing.
I have been able to get the cold back twice by toggling the off and auto switch and I got cold air, then about 2 minutes later or less, click and hot air.

It is a 1988. I have read the service manuals and I think it is the temperature control valve or the actuator.

I have some questions :

Where is the temperature control valve ? Inside behind dash or in the engine compartment ?

Where is the control valve actuator, is it on the side of the condenser ?

How do I fix or test ?

I have looked on Ecklers, Zip and others and cannot find the parts .. any help with part numbers would be appreciated

WVZR-1

Temp door failure is generally both or one of these part #'s GM 1224062 and 20007898 - They are #'s 4 & 14 respectively in this image. Do you have an FSM? No guarantees that this is the issue but it's certainly a possibility. There are substitutes for one or both. I've not looked for a very long time.



20007898 can be purchased from Corvette Central I'm quite sure. Both part #'s when searched should surface multiple threads.

Big Bird 88

WVZR-1 thanks. I got this picture off the web today and was told to look behind the dash pad. I m working the weekend so will do it Monday.

I have researched all day and I think its either the door mechanism or the heater core control valve. I found a youtube video for fixing it, but could not find the valve :-(

Here is the picture


Blend door mech, passenger side behind breadbox.

and here is the youtube video


Keep the suggestions coming, I really appreciate the assistance.

Big Bird 88

I have the auto AC control head and tried to get the codes from it. Hold down rear defrost and ^ fan button. Nothing. Is there a trick to this ? No code on the ECM, was hoping the debug on the AC would point me in the right direction.

HAD2HAV2

Big Bird, pull the Blower Control Module out. In this pic it is in the lower left corner (red wire) attached with 3 - 7mm screws. If you look down in the cavity you will see the door that closes the heater core for cold air and then the opens up the heater core for heat. With ignition ON you can move the door by adjusting the temperature on your controller from 60 to 90. Helps to have a helper so you can see the door move.


BCM is where the red wire is. Unplug it, remove the screws, wiggle it out.
The other end of the rod you see in your picture is attached to this door.

Big Bird 88

HAD2HAV2,

Thanks for the pic. So if I pull the BCM out and change the AC temp from 60 to 90 with the ignition on and the door does NOT move then this is telling me the door or the rod or the bushing is likely bad ? IF so then I pull the bread box and check the other end of that rod that's in my picture ???

I am really not mechanical but I am learning. I'm certainly not scared to spend some time working on it.

Big Bird 88

Just out of curiosity, since this seems to be related to Air accross the heater core. Would a heater core bypass be a temporary fix to return cold AC ?

Or if it is the door or rod or bushing is there a way to keep the door closed until I get parts etc ??? North Florida in the summer is HOT !!!!

Cliff Harris

It's actually the ^ temperature button (I got fooled on that too).

Changes were made over the years on how to get into diagnostic mode. Your method worked on coupes but not on convertibles because they don't have a rear defroster. See this thread for more info:

https://www.corvetteforum.com/forums...stic-mode.html

Big Bird 88

OK I Finally got the debug to work. IT was external temp and then auto fan.. apparently the setting for convertibles, even though mine is a coupe with the rear window defrost...go figure.

Well here are the codes... Can anyone help decode these for me,

Mode 1 - Program Number 35
Mode 2 - In-car thermistor 112
Mode 3 - Ambient Correction 58
Mode 4 - Blower number 55
Mode 5 - Outside Temperature thermistor 61
Mode 6 - feedback pot position 17
Mode 7 - calculated door position 55
Mode 8 - system Faults 00
Mode 9 - Software version number 55
Mode 10 - unknown 01
Mode 11 - unknown 16
Mode 12 - 86

HAD2HAV2

About this Air Blend door, if your control head is good the door should move back and forth. If the door moves freely (get a long rod and try to gently move it) then that little white clip in your pic is either broken or has come off. Cliff Harris is a great expert here on the Electronic Climate control units
.
I had 2 problems with my 88 electronic climate control system. The clip broke and the door would open and close by driving around a curve. I would have cold air then suddenly get hot air.
After I fixed the clip I discovered another problem, the AC programmer. It would not let the door move at all regardless of the temp setting from 60 to 90.

Pull the BCM first to see what is going on.

Big Bird 88

OK so when I pull the blower control motor on Monday I'll test and see if the door moves when I have the ignition in the on position and change the temperature.

If it does not work I will disconnect the negative from the battery, reconnect it and see if the door moves (recalibrates)

If the door is intact , the rod and bushing are good but it does not move, is that a head unit issue or the controller down by the drivers feet ?

There is a guy on ebay rebuilding C68 head units for $99 not sure if he does the controller unit though.

HAD2HAV2

I see you already know about the AC programmer above the gas pedal. Removing it is a royal pain. I can give you some real good tips. The problem could be either one. Keep us informed. I sent my programmer off to batee.com. Brian does great work and is turnaround is very quick. Go to his web site and look around. He also works on the head units I think. Hopefully Cliff will chime back in when you learn more about the air blend door movement.

Big Bird 88

Just from what I have read over the last day or so. Based on what I have read, if the mechanical door mechanism is intact, then it is the Head Unit or the programmer.

Either way I am going to be delving into this car on Monday deeper than I have before.

Wish I could do it sooner but I am working 12 hour night shifts between this evening and Monday AM.

Thanks again for the help, I will update this thread Monday with what I find. I'm really hoping for a simple fix, need this AC back as this is my daily driver.

I found the BATEE website, good to get a recommendation.

I welcome anyones input as I have been a corvette owner for about a month.

WVZR-1

Don't look for the valve. An '88 has no valve. '87 I believe last year for the valve. By this time you may have figured that out but, it's not been modified.

Big Bird 88

Anyone able to tell me what those codes mean ??

WVZR-1

A WAG here maybe but I'd think it likely. In '89 the access to the diagnostics changed for coupes to match the 'verts I believe and maybe yours is actually a REMAN service AC Control. If you've got it loose and/or in hand maybe the part number off of it. In the '89 FSM it mentions all fields other than MODE 08 for field service isn't relevant. I'm guessing if you called GM TECH when new with a problem they might ask relevant questions regarding the software and other BINARIES displayed.

The diagnostics for mode etc require flipping back and forth through some FSM pages. On yours MODE 8 displays a 00 so it see's no faults I'd say. Another interesting thing in the '89 FSM is that if there were 2 codes then the number displayed would be the TOTAL of BOTH error codes (the example I believe was 2 + 8 would display a 10)

There are no codes displayed for mode or temp control and those diagnostics are manual procedures. Did you get an FSM for your car? Your problem is a no-code diagnostic procedure.

Big Bird 88

Yes I guess with a 30-year-old car anything is possible. Head unit is still in the Dash. Won't be able to work on it until Monday afternoon at the earliest. Darn job getting in the way and all that :-)
I'm hoping that is the bushing thay connects the rod to the gate. That would also make sense with their not being any error codes. As the head unit still thinks it is operating correctly.
When I take the blower control out I'm hoping that I do not see the gate move also.this would again point to the pushing I think.

Thanks to you and everyone for all the help. I think by the end of Monday I should have a pretty good idea of exactly what's going on. I hope the Corvette gods are smiling down on me.

Cliff Harris

The program number is which "mode" the HVAC controller is in. It will be different for AUTO, BI-LEV, etc.
The in-car thermistor and outside temperature thermistor vary with the temperature and will range from 0 to 255.
The ambient correction speeds up the fan if it's hot outside.
Blower number is the fan speed.
Feedback pot position is the reading from the potentiometer on the bottom of the blend door motor assembly. Calculated door position goes with this and is the relative position of the mechanism between 0 and 255.
System faults are bitmapped and add together. The defined ones are:
00 - no faults
01 - feedback potentiometer open
02 - ambient sensor open
04 - ambient sensor shorted
08 - in-car sensor open
16 - in-car sensor shorted

As you can see, the diagnostics are pretty stupid and don't really tell you a lot about what is wrong.

Big Bird 88

Thanks Cliff that sure beats not knowing.

My plan after checking the gate through the blower control motor is to remove the dash pad so I can remove the breadbox so that I can see the arm and the bushing assembly.

I think I'm on the right track going that way I don't think this is something I want to approach from the underneath the dash?

Big Bird 88

Had2hav2
I would appreciate those tips on the AC program of removal just in case I need them Monday or Tuesday. I'm keeping my fingers crossed I'm hoping that it is a gate issue but I'd like to be prepared just in case. Thanks for the info